Delirium is a frequently occurred cerebral complication in elderly patients after surgery, and its occurrence is associated with worse outcomes. Sleep disturbances is considered to be one of the most important risk factors of postoperative delirium. Previous studies showed that, for elderly patients admitted to the ICU after surgery, low-dose dexmedetomidine infusion improved the quality of sleep and decreased the incidence of delirium. The investigators hypothesize that, for elderly patients after cancer surgery, dexmedetomidine supplemented analgesia can also decrease the incidence of delirium, possibly by improving sleep quality. The purpose of this multicenter, randomized controlled trial is to investigate the impact of dexmedetomidine supplemented analgesia on the incidence of delirium in elderly patients after cancer surgery.